Conventionally, ophthalmic lenses have been used as optical elements for correction in the human optical system and as alternative optical elements after crystalline lens extraction. Among them, contact lenses applied to the eye and intraocular lenses inserting therein have been used extensively because they provide a wide vision by being directly used for the human eye while reducing the uncomfortable feeling in seeing objects.
Meanwhile, there are increasing number of people in recent years who reached the presbyopic age and continue to wear contact lenses. Since such seniors who suffer from presbyopia have their focal functions deteriorated, they develop a symptom of hardly being able to focus on objects nearby. Therefore, presbyopia patients will need multifocal contact lenses that allow them to focus on nearby objects, too. Also, since patients who underwent a cataract surgery have their crystalline lens removed that used to adjust the vision, they still have symptoms resulting in difficulties in seeing nearby objects even if intraocular lenses are inserted in their eyes. It is becoming necessary for such intraocular lenses to have a multifocal function realized by multiple focal points. Thus, needs for multifocal lenses are increasingly growing in recent years reflecting our aging society.
As methods of producing such multifocal ophthalmic lenses, there have been known a refraction-type multifocal lens that forms multiple focal points based on the refraction principle and a diffraction-type multifocal lens that forms multiple focal points based on the diffraction principle. In the latter mentioned diffraction-type multifocal lens, the optical part of the lens is provided with a plurality of diffraction structures formed concentrically, and multiple focal points are formed by the effect of mutual interference between light waves that pass through said multiple diffraction structures (zones). Therefore, such lenses have an advantage of being able to set a lager lens power while minimizing the lens thickness as compared to refraction-type lenses that generate focal points by the refraction effect of light waves at the refraction interface, which is a boundary of different refractive indices.
Generally speaking, the diffraction-type multifocal lens has a diffraction structure where the pitch of diffraction zones gradually gets smaller as it moves from the center toward the periphery according to a certain rule called ‘Fresnel pitch,’ and the 0th order diffracted light and first-order diffracted light generated from said structure are used to produce multiple focal points. Usually, the 0th order diffracted light focuses for far vision while the first-order diffracted light focuses for near vision. By providing such a distribution of diffracted light, a bifocal lens can be produced having focal points for far and near visions.
In recent years, needs for a trifocal lens have been pointed out that can provide a focal point not only on the near side but also at an intermediate position between far and near ranges to be used for patients of advanced presbyopia or cataract patient who had the crystalline lens extracted and an intraocular lens inserted. Examples of such a diffraction-type multifocal lens that can generate three focal points include those disclosed in Japanese Unexamined Patent Publication No. JP-A-2010-134282 (Patent Document 1) and Japanese Unexamined Patent Publication No. JP-A-2010-158315 (Patent Document 2) and so forth. Both of these examples are based on the Fresnel pitch rule but their relief configurations in the diffraction zone are varied.
However, the diffraction-type ophthalmic lens has a problem of easily generating multiple concentric circles of light around the light source when the light source is viewed by an eye in the distance at night no matter whether a bifocal lens or a trifocal lens is used. This circle of light usually called ‘halo’ tends to appear around a point light source such as a street light in the distance or a motor vehicle headlight or the like, which causes a problem of deteriorated visibility at night using the ophthalmic lens. The halo is one of the phenomena reflecting the imaging characteristics of multifocal lenses, especially those called the simultaneous perception-type, and the cause of the halo formation can be explained as follows:
In case of an ideal monofocal lens with no aberration, light from far distance passes through the lens and focuses an image at a given focal point position so as to intensify the amplitude of light waves each other to the maximum extent (FIG. 48A). In that process, the intensity distribution at the image plane shows a simple pattern of a main peak at the center thereof with only very small side lobes defined by the Airy radius existing around it (FIGS. 48B, 48C). FIG. 48C is a magnified view of FIG. 48B. Therefore, when a light source is viewed from far away, an image is formed with no halo that reflects such intensity distribution (FIG. 48D).
Meanwhile, a diffraction-type multifocal lens having two focal points for far and near visions is designed in such a way that the light from far distance produces an image at the far focal point position so as to maximize the amplitude of light waves each other, while intensifying the amplitude of each other at the near focal point position, too. Light from far distance forms the main peak centered around the image plane at the far focal point, whereas light waves intensified each other at the near focal point position diverge thereafter to reach the image plane at the far focal point (FIG. 49A). At a first glance as shown in FIG. 49B, there seems to be only one main peak on the image plane at the far focal point, but as shown in the magnified view of FIG. 49C, a group of small peaks can be observed. As mentioned above, these peaks were formed by the light components focusing at the near focal point to be mixed in the far focal point image plane as a kind of stray light. Thus the intensity of the group of small peaks is very small compared to that of the main peak, but even light with the smallest intensity can be conspicuous in the night environment with dark background, and further, the image can be better detected by the retina with high visual sensitivity to have it perceived as a halo (FIG. 49D). The group of small peaks will hereinafter be referred to as ‘side-bands (peaks).’
Other background arts propose a solution to the halo problem addressed regarding the diffraction-type multifocal ophthalmic lens. Japanese Domestic Publication of International Patent Application No. JP-A-2000-511299 (Patent Document 3), for example, discloses a method of smoothly reducing the height of the diffraction zone in the periphery in a diffraction structure composed of one form of diffraction zone called ‘echelette’ in order to reduce the halo as well as a function that determines the change in height. Said method tries to reduce the amount of energy distributed to the near focal point as it moves toward the periphery and reduce the halo as a result. However, in the background art mentioned above, the amount of energy distributed to the near side needs to be much lowered in order to reduce the halo to an imperceptible level, in which case there is a problem that the visibility of near objects is significantly deteriorated.
Also, Japanese Unexamined Patent Publication No. JP-A-2007-181726 (Patent Document 4) discloses a multifocal ophthalmic lens that blocks or reduces the transmission of blue light and/or near UV light in order to eliminate glare and halo. In such background art, scattering of light is considered to be the cause of the halo and glare, and it is assumed that the halo and glare can be reduced by preventing the transmission of short-wave light that is subject to scattering. However, the halo is attributed more to the intrinsic behavior of light in generating a near focal point rather than the scattering of light, and therefore, the background art does not bring a basic solution to the problem although some ancillary effects can be expected. Also, since the imaging mechanism of the trifocal diffraction-type ophthalmic lens described in the above background art is no different from that of the bifocal lens and it is inevitable to have the light form an image at multiple focal points mixed in the far focal plane as stray light, the problem of halo described above inherently exists. For that reason, there does not yet exist a diffraction-type multifocal lens such as a bifocal or trifocal lens with the halo reduced to a reasonable level.